The accretion disc in black hole binary systems (BHB) is unstable at
the point where Hydrogen goes from being predominantly neutral to
ionised. This gives rise to dramatic, transient outbursts in which
the reservoir of material built up in the outer disc during
quiescence can accrete down onto the black hole. The peak luminosity
of the outburst is set by the mass in the quiescent disc and the
time taken for this to accrete. Both these depend on the size of the
disc taking part in the outburst, which is set by the binary
separation. Hence there is a link between orbital period and peak
luminosity, with short period systems showing smaller peak outburst
luminosities ([1]}; [2]}; [3]};
[4]} hereafter DGK07).
